They have been doing that for years. A long time ago there was a submission here on HN about some Ivy League college urging college applicants to provide something other than a Gmail address because application-related emails (including acceptances!) would frequently not get through.

Started? My Gmail spam filter has been broken for several years now. Extremely obvious spam regularly ends up in the primary inbox and critical, fully legitimate messages get buried in spam. It’s a major reason why I left for another provider.